In recent years, large-screen projection-type displays have been enthusiastically developed for the purpose of seeing videotapes at home and of presentation. As a conventional large-screen display, a projector equipped with a TFT (Thin Film Transistor) liquid crystal panel or a CRT (Cathode Ray Tube) is used, and for presentation, a slide projector and an overhead projector are used. However, since they provide very dark brightness of screens, in the case where the above-mentioned projectors are used, it is necessary to darken a room. Therefore, the above-mentioned projectors are unsuitable for seeing videotapes in a light room or for taking notes while presentation.
Therefore, in order to improve brightness of a screen, a liquid crystal light valve is used in the above-mentioned projectors. For example, U.S. Pat. No. 4,019,807 issued 1977 by Boswell et al. describes a photoaddressed liquid crystal light valve which uses cadmium sulfide (CdS) as a photoconductor layer, cadmium telluride (CdTe) as a light blocking layer, MgF/ZnS which is a dielectric multilayer as a light reflecting layer. However, since the CdS is used for the photoconductor layer, a responding speed of an element is slow and reproducibility is inferior. Therefore, U.S. Pat. No. 4,799,773 issued 1989 by Sterling describes a photoaddressed liquid crystal light valve which uses hydrogenated amorphous silicon (a-Si:H) providing a high responding speed of an element and excellent reproducibility as a photoconductor layer, CdTe as a light blocking layer, and SiO.sub.2 /TiO.sub.2, which is a dielectric multilayer as a light reflecting layer. However, this liquid crystal light valve has problems that the CdTe as the light blocking layer does not adhere well to the hydrogenated amorphous silicon as the photoconductor layer, and that light blocking properties of the CdTe are not sufficient.
Therefore, for example, a reference 1 (U.S. Pat. No. 5,084,777 by David E. Slobodin Greyhawk Systems, Inc.) discloses a photoaddressed liquid crystal light valve whose purpose is a display of high brightness which uses hydrogenated amorphous silicon (a-Si:H) as a photoconductor layer, hydrogenated amorphous silicon germanium (a-SiGe:H) providing excellent adhesion to the photoconductor layer and excellent light blocking properties as a light blocking layer.
As shown in FIG. 10, for example, the liquid crystal light valve of the reference 1 is composed of a liquid crystal layer 107, a light reflecting layer 104, a light blocking layer 103 and a photoconductor layer 102, and has a sandwich construction such that each layer is put between light transmitting substrates 100a.100b made of glass, etc. where transparent electrically conductive films 101a.101b have been formed. The liquid crystal layer 107 modulates intensity of a read light 110 according to a change in a voltage to be applied. The light reflecting layer 104 is composed of a dielectric multilayer film which reflects the read light 110. The light blocking layer 103 is composed of hydrogenated amorphous silicon germanium which blocks a transmitted light from the light reflecting layer 104. The impedance of the photoconductor layer 102 changes according to intensity of writing light 109, and is composed of hydrogenated amorphous silicon which controls the voltage applied to the liquid crystal layer 107.
In addition, the liquid crystal layer 107 is formed such that liquid crystal is sealed in a gap which is formed by providing spacers 106.106 between alignment films 105a.105b. The alignment film 105a formed on the light reflecting layer 104 and the alignment film 105b formed on the transparent electrically conductive film 101b align liquid crystal molecules. Moreover, The transparent electrically conductive films 101a.101b are connected to a driving power source 111 which generates an alternating voltage.
In the above liquid crystal light valve, when an image signal is not inputted to the photoconductor layer 102 by the writing light 109 (in a dark state), the photoconductor layer 102 has high impedance, but when the image signal is inputted to the photoconductor layer 102 by the writing light 109 (in a light state), the photoconductor layer 102 has low impedance due to a photoconductive effect. As a result, the voltage to be applied to the liquid crystal layer 107 exceeds a threshold voltage, and an alignment state of the liquid crystal layer 107 is changed. A reflected light of the reading light 110, in which intensity changes according to the alignment state, is projected on a screen through a polarizing beam splitter, etc. so as to be taken out as an image signal.
In addition, a projection-type liquid crystal display apparatus which uses a liquid crystal light valve has an arrangement shown in FIG. 11. In this projection-type liquid crystal display apparatus, an image which has been formed by a CRT 122 is written to a liquid crystal light valve 123 by a fiber plate, etc., whereas the reading light 110 emitted from a reading light source 121 enters the liquid crystal light valve 123 through an optical lens 125a and a polarizing beam splitter 124. After the reading light 110 is modulated in the liquid crystal light valve 123 according to the image, the reading light 110 passes through the polarizing beam splitter 124 and an optical lens 125b so that its image is projected on a screen 126.
Incidentally, in next-generation media represented by hi-definition television, its brightness and its resolution have been developed higher, and accordingly, in the projection-type liquid crystal display apparatus, its brightness and resolution have been developed higher.
For this reason, in order to improve the brightness of the projection-type liquid crystal display apparatus higher, it is necessary to develop brightness of the reading light source 121, so it is necessary to increase output of the lamp. In this case, since a temperature of a whole system of the projection-type liquid crystal display apparatus rises, a temperature of the liquid crystal light valve also rises to 40.degree. C.-60.degree. C.
In the reference 1, a relationship among electric conductivity (hereinafter, referred to as conductivity), light blocking properties and a temperature of the light blocking layer is represented by a formula (1), and the conductivity is optimized by using Eopt in a range of 1.0&lt;Eopt&lt;1.5. EQU .sigma.=.sigma..sub.0 exp (-Eopt/2kT) (1)
.sigma.: conductivity (S/cm) PA0 Eopt: optical gap (eV) PA0 k: Boltzmann's constant 8.62.times.10.sup.-5 (eV/K) PA0 T: temperature (K) PA0 .sigma.: conductivity (S/cm) PA0 Ea: activation energy (eV) PA0 k: Boltzmann's constant 8.62.times.10.sup.-5 (eV/K) PA0 T: temperature (K)
However, in general, as is clear from FIG. 14 and the formula (1), as to an amorphous semiconductor thin film, even when the Eopt is optimized, its conductivity rises due to a rise in temperature, so its resistivity is lowered according to the rise in temperature. For this reason, in the case where a photoaddressed liquid crystal light valve is produced by using a light blocking layer having a condition of 1.0&lt;Eopt&lt;1.5 mentioned in the reference 1, there arises a problem that the resolution is lowered because as shown in FIG. 12, the temperature rises, as time passes.
In addition, a reference 2 (Abstracts of one lecture of the 40th lecture meeting held by Applied Physics association, Spring 1993, No. 29p-ZD-1 "Preparation and Characterization of a-SiGeC:H Light Absorption Layers for Liquid Crystal Spatial Light Modulators, NGK INSULATORS, LTD., Science & Technical Res. Lab., NHK) describes a photoaddressed liquid crystal light valve which uses BSO as a photoconductor layer and hydrogenated amorphous silicon germanium carbon (a-SiGeC:H) as a light blocking layer. However, the reference 2 does not describe that temperature properties were improved according to a relationship between conductivity and temperature of the light blocking layer.
Therefore, it is necessary to provide the light blocking layer where sufficient light blocking properties are given by relieving the temperature properties of resistivity of amorphous semiconductor and by repressing a drop in the resolution of the liquid crystal light valve as small as possible also under high temperature.
In addition, the temperature properties of the light blocking layer will be described based upon a formula (2) by using activation energy Ea of the light blocking layer. Here, the formula (2) shows a generally relational formula between the conductivity and the activation energy. EQU .sigma.=.sigma..sub.0 exp (-Ea/kT) (2)
As mentioned above, as shown in FIG. 13, the lowering of the resolution of the liquid crystal light valve is caused by a rise in the conductivity of the light blocking layer due to the rise in temperature.
In addition, as is clear from the formula (2), the rise in the conductivity due to the rise in temperature is represented by the activation energy Ea. Ideally, when Ea =0 eV, the conductivity has no temperature properties, but it is not practical.
In addition, the reference 1 specifies a relationship between the optical gap Eopt and activation energy Ea as Ea.congruent.Eopt/2. According to this relationship, the relationship between the optical gap Eopt and the activation energy Ea is 1:1, so for example, when Eopt=1.1 eV, Ea=0.55 eV, and when Eopt=1.3 ev, Ea=0.65 eV. Therefore, the larger the Eopt is, the larger Ea becomes, in other words, the larger the Eopt is, the higher conductivity rises according to the rise in temperature, thereby lowering the resolution remarkably.